Manufacture of nonwoven fabrics



Patented Sept. 28, 1943 MANUFACTURE OF NONWOVEN-FABRICS George L.Schwartz, Wilmington, Del., assignor to E. I. du Pont de Nemours &Company, Wilmington, Del.. a corporation of Delaware No Drawing.Application August 23, 1940,

' Serial No. 353,839

8 Claims.

This invention relates to non-woven fabrics of high wet strength andmore particularly to porous, non-woven fabrics containing caustic alkalitreated polyvinyl alcohol. v

Unwoven porous fabrics usually comprise a major amount of cellulosefibers to give strength with a minimum but considerable amount of woolfibers to produce bulk and porosity. There are two main types,consisting of those from fibers that are long enough to operate on atextile carding machine and those that are too short for use in thisequipment but which are adapted to oper-' ate in mechanisms that aredesigned to convert fibrous materials into paper. The long fiber type isusually converted into a felt by combining a number of laps of cardedfibers in the dry state and felting by means of heat and pressure or bymechanical treatment in aqueous systems that promote integration of thefibers into a more or less felted condition. The short fiber type isusually converted into a felt by the wet method which consists inpreparing the fibrous ingredients in a paper beater, Jordan engine orboth, and then forming the felt on a wire screen, pressing out excesswater and drying by passage over heated metal cylinders. Recently a typeof cellulose fibers of paper making length has been developed for thiswet method of felt formation by treating chemical wood pulp fibersprevious to felt formation with a strong caustic alkali solution to givethem a crinkle. Felts of the short fiber type have been prepared also bytreating a waterleaf web of paper fibers with a strong caustic alkalisolution, festooning the alkali saturated sheet in air to dry slowly andconverting the caustic alkali to alkali carbonate crystals, washing andthen drying. This method forms a product that is very porous, soft,water-absorbent and of moderate wet strength, but it is not strongenough in the wet state to compete with materials in its cost category.I Felts have been prepared by treating waterleaf webs of paper fiberswith strong alkali solutions at low or high temperatures and removingthe alkali promptly by washing. The process cannot be operatedeconomically because the sheet is too weak to wash effectively at highspeed, and the product is too weak for most uses.

This invention has as an object the development of a web that hassuflicient strength in strong caustic alkali treated liquor to permithigh speed handling. Another object is to provide a process for themanufacture of non-woven fabrics of high wet strength. Another object isto provide a process for the manufacture of a soft, porous, non-woven.felt-like product with suflicient wet strength so it may be handledwithout tearing in use. Another object is to produce a new non-wovenfabric. Still another object is to provide a method for the manufactureof a soft, porous, felt-like product that does not lint. Other objectswill be apparent from the following description of the invention.

v These objects are achieved by applying to a non-woven web of fibers asolution of polyvinyl alcohol, impregnating the web with a solution ofat least 0.5 normal concentration of an alkali metal hydroxide or abasic reacting alkali metal salt, e. g. those salts giving an aqueoussolution having a pH greater than 7, washing free from alkali anddrying.

The webs may be prepared by any of the wellknown methods of preparingwebs from fibers of the nature covered by this invention. When webs areused that are made from fibers of carding length, they are passedbetween screens, and while in this condition, they are passed through asolution of polyvinyl alcohol and then passed between rolls to removeexcess polyvinyl alcohol solution, dried and wound into a roll. They areI then passed without any support through a bath of an aqueous sodiumhydroxide solution of 0.5 normal concentration to 12.0 normalconcentration at 10 to +100 0., washed free from alkali and dried bypassage over steam heated cylinders or by passage through a heatedchamber in a stretched condition or as festoon loops. When webs are usedthat are made from fibers of paper making length, the polyvinyl alcoholsolution is applied by spray or roll to the sheet in the wet presssystem, passed over drying cylinders to attain a moisture content of20-10 per cent, passed through a solution of sodium hydroxide of atleast 0.5 normal concentration at -10 to +100 C., washed free fromalkali and dried by passage over drying cylinders. The preferredtemperatures of treatment are 5 to +40 C.

The fibers of carding length may be cotton, flax, ramie, jute,pineapple, musa textilis, banana or viscose staple, in the bleached orunbleached condition. The fibers of paper making length may be sulfite,sulfate, soda, rag, rope, or ground wood either in bleached orunbleached condition. The polyvinyl alcohol should be of the hot watersoluble type with an intrinsic viscosity as a 4 per cent aqueoussolution at 25 C. of 4 to 100 centipoises and a saponification valueunder 100. Polyvinyl alcohols with intrinsic viscosities of 10 to andwith saponification values below 20 are preferred. The caustic alkalimay be sodium,

potassium or lithium hydroxides. Alkali metal salts which are basicreacting in aqueous solutions, e. g., which form aqueous solutionshaving a pH greater than seven, are also useful and include sodium,potassium or lithium carbonates, trlsodium phosphate, sodium silicate,and the like. The sodium, potassium or lithium alkalies may contain,besides the hydroxides, considerable amounts of carbonates or chlorides.

The following examples will serve to illustrate this invention withoutlimiting it thereby:

Example I 0.037" thick and consists of bleached sulfite high alphacellulose from wood. The polyvinyl alcohol has a specific viscosity of23 centipoises and a saponification value of 8. The sheet is then passedover a knife edge under tension to increase the pliability. It is soft,rapidly wetted by water, tough when dry or wet, and it does not "lintwhen rubbed on a wet smooth glass plate. Its wet strength is 1.75 poundsfor a strip 0.5" wide. A control sheet that is sprayed with water onlyand dried before treatment disintegrates completely on passing throughthe sodium hydroxide solution. The sheet containing polyvinyl alcoholbecomes tough and rubber-like at the instant that it is impregnated withthe sodium hydroxide solution, and it retains this strength throughoutthe washing and drying treatments.

Example II To a web of kraft fibers a 5 per cent solution of polyvinylalcohol is applied in the stage just ahead of the drier system insufficient amount to give 3 per cent polyvinyl alcohol, based on thefibers. The sheet is passed over the steam heated drier system, passedthrough a 20% aqueous solution of sodium hydroxide at 25 C., washed freefrom alkali, passed over a drier system and then over a knife edge tosoften. The sheet is much softer than ordinary waterleaf kraft, feelslike a chamois skin when wet, but is considerably denser than theproduct of Example 1. Its wet strength from a strip 0.5" wide is 2.62pounds in contrast to wet strength of 0.04 pound for a sample of thesame paper that is given the same treatment with aqueous sodiumhydroxide solution but with the omission of the polyvinyl alcoholapplication step. Basis weight of sheet is 50 pounds per ream. Thepolyvinyl alcohol used has an intrinsic viscosity as a 4% solution at 25C. of 78 centipoises and a saponification value of 14.

Example III A web is prepared by carding a good ordinary grade of 1"staple cotton fibers and combining six of these carded webs into acontinuous web with the fibers of alternate layers aligned in diagonaldirections to the main direction of the web. After compressing andpassing between metal screens, the web is impregnated with an 8%solution of polyvinyl alcohol having an intrinsic viscosity of 54centipoises and a saponification value of 6. The web is compressedsuffioiently to leave enough polyvinyl alcohol solution to make 10%,based on the fibers, and dried. The dry web is then passed withoutsupport through a 24% aqueous solution of potassium hydroxide at 30 C.,washed free rrom alkali, sprayed with a solution of glycerol insuflicient amount to make 05% based on the fibers, dried and passed overa knife edge to increase the pliability. Its wet strength from a strip0.5 wide is 11.8 lbs. in contrast to a wet strength of 1.1 lbs. for asample of the same material that is given the same treatment withaqueous potassium hydroxide solution with omission of the polyvinylalcohol treatment.

Example IV Viscose rayon staple of 2" length is carded into a continuousweb, and four of these webs are combined into a mat and treated withpolyvinyl alcohol as in Example III except that it is squeezed to leave15% polyvinyl alcohol based on fibers. The dried web containingpolyvinyl alcohol is passed through a 20% solution of trisodiumphosphate at 60 C., washed free from sodium phosphate and dried. Theproduct is a porous, soft felt with a wet strength of 12.3 lbs. for astrip 0.5" wide in contrast to a wet strength of 0.1 lb. for a sample ofthe same material that is given the same treatment with aqueoustrisodium phosphate solution and with omission of the polyvinyl alcoholtreatment.

Example V Thirty parts of a middling grade of cotton fibers are blendedwith 70 parts of fiber glass of about the same diameter and length bypicking and carding into laps, and these are combined into a mat as inExample III. This mat is supported between wire screens and passedthrough a 5% aqueous solution of polyvinyl alcohol, having an intrinsicviscosity as a 4% solution at 25 C. of 42 centipoises and asaponificatlon value of 24, passed between rolls to squeeze out excesssolution and leave enough to make 5% polyvinyl alcohol based on the dryfiber weight, dried and wound into a roll. This dried mat is'thenpassed, without support, into a solution at 25 C. containing sodiumhydroxide 15 parts, sodium chloride 15 parts and water 70 parts, washedfree from alkali and dried. The product is porous, of high dielectricvalue, and has a wet strength for a strip 0.5" wide of 5.2 lbs. incontrast to a wet strength. of 0.7 lb. for a sample of the same materialthat is given the same treatment with aqueous sodium hydroxide andsodium chloride solution with omission of the polyvinyl alcoholtreatment.

Example VI To a web of unbleached sulfite fibers a 5% solution ofpolyvinyl alcohol identical with that recorded in Example I is appliedin the stage just ahead of the drier system in sufiicient amount to give4% based on the fibers. The sheet is passed over steam heated cylindersto dry, passed through a solution of sodium carbonate (25% NazCOa), andit is then festooned at a temperature below 25 C. and at a relativehumidity corresponding to at least 65% at 25 C. until the sheet is dry.The sheet is then washed free from alkali and dried. It is soft, bulkierthan the product of Example II and lint free when used to wipe a wetglass surface.

The webs which have been treated with a polyare dried, when the treatingsolutions are below C. or at higher temperatures when they contain asmall amount of borax. Methods of treating the webs with alkalisolutions are not restricted to spraying or dipping. Just enough alkalisolution may be applied to the web to wet it entirely, or an excess ofalkali solution may be added, and then the excess may be removed bysqueezing, etc. Treatment with alkali solutions at temperatures below 10C. produces the greatest sheet porosity and wet strength. Treatments atroom temperature are in many cases most economical, and treatments attemperatures up to 100 C. are effective when the time is limitedsufiiciently to avoid degradation. The sheets develop the maximumstrength effect just as soon as the alkali solution penetrates themthroughout, and this time of penetration varies from less than onesecond to several minutes, depending on the density of the originalwebs, temperature and concentration of the alkali solution, ratio of thepolyvinyl alcohol to fibers, and wetting agents employed. The webs maybe washed countercurrently in cold dilute solutions of the same materialas that used for treating or in water at room or higher temperatures,although room temperature is preferred. Washing rate is increased bypassing the web between squeeze rolls after each addition of washliquors.

The caustic alkali solution can be made from commercial grades oflithium, sodium, or potassium hydroxides which may contain considerableamounts of the chlorides or carbonates and smaller amounts of othercommon impurities. In the case of the basic reacting salts the sodiumsilicate solutions may vary in ratioof the sodium to $102, but those ofthe high sodium ratio are most effective. Trisodium phosphate of allcommercial grades is useful. Borax in small amounts may be added to anyof the alkali solutions to increase the-speed of setting" of thepolyvinyl alcohol in the fabric. The strength of the alkali solution mayvary according to the results desired. Solutions of from 0.5 normalconcentra= tion up to the upper limit of solubility at the temperatureused of the alkali hydroxide or basic reacting salt may be employed. A0.5 normal solution of NaOH is a 2% solution by weight. Twelve normal,or 48% concentration by weight, is about the upper limit of operabilityfor NaOH since compositions containing higher percentages of NaOH are nolonger sufiiciently fluid at the lower temperatures of treatment.

The fibers may be of any length that is operable on a carding machine orof any length that is operable in paper making machines. They may befibers of animal origin, natural cellulose fibers or synthetic fiberssuch as staple from viscose, cellulose ethers or polymeric resins.

Fibers of animal origin may be usedwith the alkali solutions only at lowtemperatures and only for brief periods of treatment. Mixtures of thevarious fibers may be used to obtain certain desired effects.

:The polyvinyl alcohols that are useful are tough, water-whiteresin-like materials which are usually obtained by the hydrolysis ofpolyvinyl esters, such as polyvinyl acetate. They are difficultlysoluble to insoluble in cold water, but all of them are soluble in hotwater and remain dissolved when the solution is cooled. Althoughpolyvinyl alcohol maybe made in a number of modifications of variousdegrees of polymerization, those only are useful that have an intrinsicviscosity as a 4% aqueous solution at 25' C. of

4 to 100 centipoises and saponification values below 100. The, term"polyvinyl alcohol is used herein and in appended claims to designategenerically pure polyvinyl alcohol and also such partial derivativesthereof that contain a sufllcient number of free hydroxyl groups torender the composition soluble in hot water.

Various softeners may be applied to the sheet after it is washed freefrom alkali and before or after drying. Useful softeners are formamide,ethanol formamide, ethanol acetamide,

glycerol, invert sugars, certain hydroscopic salts,

such as sodium lactate, and the higher fat alcohol sulfates. The ratioof softeners to polyvinyl alcohol may varyin a wide range, but 2 to 20%,based on polyvinyl alcohol, represents the preferred range.

The ratio of polyvinyl alcohol to fibrous ingredients may vary from 0.1to 100% based on dry fiber weight. The preferred range is 0.5 to

The penetration of the causticalkali solution into the fabric may beassisted by addition of small amounts of surface active substances ofthe type of mercerizing assistants and wetting agents for mercerizingsolutions. Compounds of this type are described in U. S. No. 2,033,125'

and in U. S. No. 2,033,819.

The products are useful as bases for coating with pyroxylin, rubber,drying oils-and resin compositions such as in the manufacture ofsynthetic leather. They are useful also for purposes Where naturalchamois are employed, for certain items of clothing, pillow slips,towels and other purposes where high degree of water absorption,softness, high wet strength and low cost are desirable.

As many apparently widely different embodiments of this invention may bemade without departing'from the spirit and scope thereof, it is to beunderstood 'that I do not limit myself to the specific embodimentsthereof except as defined in the appended patent claims.

I claim:

1. The process for the preparation of a soft, porous, felt-like,non-woven web of fibers having a high wet strength from a soft, porous,feltlike, non-woven. web of fibers of low wet strength which comprisesapplying to the said low wet strength non-woven web of fibers polyvinylalcohol having an intrinsic viscosity of 4 to centipoises as a 4%aqueous solution at 25 C. and having a saponification value below 100,and

then impregnating the web with an aqueous solution of at least 0.5normal concentration of a compound of the class consisting of alkalimetal hydroxides and basic reacting alkali metal salts.

2. The process for the preparation of a soft, porous, felt-like,non-woven web of fibers having a high wet strength from a soft, porous,feltlike, non-woven web of fibers of low wet strength which comprisesapplying to the said low wet strength non-woven web of fibers polyvinylalcohol having an intrinsic viscosity of 4 to 100 centipoises as a 4%aqueous solution at 25 C. and having a saponification value below 100,and

'then impregnating the web with an aqueous solutionof at least 0.5normal concentration of sodium hydroxide.

3. The process for the preparation of a soft, porous, felt-like,non-woven web of fibers having a high wet strength from a soft, porous,feltlike, non-woven web of fibers of low .wet strength which comprisesapplying to the said low wet strength non-woven web of fibers apolyvinyl alcohol solution, then impregnating said web with an aqueouscaustic alkali solution of from 0.5 to 12.0 normal concentration,washing same free from alkali and drying.

4. The process for the preparationof a. soft,

porous, felt-like, non-woven web of fibers having a high wet strengthfrom a soft, porous, feltlike, non-woven web of fibers-of low wetstrength which comprises applying to the said low wet strength non-wovenweb of fibers a polyvinyl alcohol solution, said polyvinyl alcoholhaving an intrinsic viscosity of from 4 to 100 centipoises when in a 4%solution in water at 25. C. and having a saponification value below 100,then passing said web through an aqueous solution of at least 0.5 normalconcentration -of a compound of a class consisting of alkali metalhydroxides and basic reacting alkali metal salts at a temperaturebetween-40 C. and +100 C.,

washing said treated web free from alkali and.

drying same.

5. The process for the preparation of a soft, porous, felt-like,non-woven web of fibers having a high wet strength from a soft, porous,feltlike, non-woven web of fibers of low w'et strength which comprisesapplying to the said low wet strength non-woven web of fibers of papermaking length an aqueous solution of polyvinyl alcohol having anintrinsic viscosity of from 4 to 100 centipoises when in a 4% solutionin water at 25 C. and having a saponification value below 100, while thesaid web is in the wet press system, then drying same so as to obtain amoisture content of from to 20% and passing said dried web through asolution of sodium hydroxide of at least 0.5 normal concentration at atemperature between 10 C. and +l00 C., washing the web free from alkaliand drying same.

6. As a new article of commerce a soft, porous,

felt-like, non-woven web of fibers having a high wet strength containingpolyvinyl alcohol and obtainable by applying to an untreated soft,porous, felt-like, non-woven web of fibers having a low wet strengthpolyvinyl alcohol having an intrinsic viscosity of 4 to 100 centipoisesas a 4% aqueous solution at 25 C. and having a saponification valuebelow 100, and then impregnating this web with an aqueous solution of atleast 0.5 normal concentration of a compound of the class consisting ofalkali metal hydroxides and basic reacting alkali metal salts.

7. As a new article of commerce a soft, porous, felt-like non-woven webof fibers having a high wet strength containing polyvinyl alcohol andobtainable by applying to an untreated soft, po-

rous, felt-like non-woven web of fibers having a low wet strengthpolyvinyl alcohol having an intrinsic viscosity of 4 to 100 centipoisesas a 4% aqueous solution at 25 C. and having a saponification valuebelow 100, and then impregnating this web with an aqueous solution of atleast 0.5 normal concentration of sodium hydroxide at between 10 and+100 C. and then washing the web free from alkali.

8. The process for the preparation of a soft, porous, felt-like,non-woven web of fibers having a high wet strength from a soft, porous,felt like, non-woven web of fibers of low wet strength which comprisesapplying to the said low wet strength non-woven web of fibers polyvinylalcohol having an intrinsic viscosity of 10 to 80 centipoises as a 4%aqueous solution at 25 C. and having a saponification value below 20,then drying the web so as to obtain a moisture content of from about 10to about 20%, then impregnating the web with an aqueous caustic alkalisolution of from 0.5 to 12.0 normal concentration at a temperature offrom about 5 C. and about +40 C. and then washing said web free fromalkali and drying.

GEORGE L. SCHWARTZ.

